In order to improve the S/N ratio in an FM demodulating system, heretofore a narrow band tracking filter, whose central frequency is made to follow the instantaneous frequency of the FM wave, has been used. FIG. 4 shows an example of the prior art FM demodulating system and FIG. 5 indicates noise distribution in the output signal of different parts thereof. In FIG. 4, reference numeral 1 is a 90.degree. phase shift circuit (differentiation circuit); 2 is a limiter; 3 is a phase comparator; 4 is a loop filter (LPF); 5 is a narrow band tracking filter; and 6 is a limiter. The FM signal frequency-converted into the intermediate frequency by the front end is advanced in the phase by .pi./2 by the differentiation circuit 1 and the amplitude variable component is removed therefrom by the limiter 2. Therefore it is inputted in the phase comparator 3 through one input terminal as the reference signal for the phase comparison. Further, the FM signal stated above is inputted in the narrow band tracking filter 5, whose central frequency follows the instantaneous frequency of the inputted FM signal. The signal is inputted in the phase comparator 3 through the other input terminal through the limiter 6 after having removed noise outside of the relevant frequency band by means of the narrow band tracking filter 5. Then a phase error signal is obtained by the phase comparator 3. The phase error signal thus obtained is inputted in the loop filter 4 and outputted from the loop filter 4 as the demodulated signal. In addition, the demodulated signal thus obtained is used as a control signal for the narrow band tracking filter 5.
In the prior art system described above, since the narrow band tracking filter 5 passes only the signal in a frequency band having the necessary instantaneous spectral width of the FM signal, the noise outside of the band can be completely removed (FIG. 5 .circle., in which BW' represents the band width of the tracking filter 5). However the signal passing through the differentiation circuit 1 and the limiter 2, from which noise is not removed, is inputted in the phase comparator as it is (FIG. 5 .circle., in which BW represents the band width of the IF filter). That is, since noise is removed only from the signal coming from the limiter 4 in the two signals inputted in the phase comparator 3, the noise removal in the sum of the two signals, which have passed through the limiters 2 and 4, is a half (6 dB) outside of the pass band of the narrow band tracking filter 5 (FIG. 5 .circle.). No noise is removed within the ass band of the narrow band tracking filter 5 (FIG. 5 .circle.). Consequently the prior art system had a drawback that improvement relating to the noise is about a half (6 dB) and that the S/N ratio is not improved satisfactorily.